Microstructure Analysis of the Rust Developing at Steel-Concrete Interface

Abstract

The rust distribution and the crack expansion in the deteriorated concrete were studied through the macroscopic and microcosmic method. The results show that the corrosion of steel bar in the concrete is non-uniform even though by Galvanostatic Method. The crack appears earlier and expands quicker at the thin concrete cover than others side and the short crack filled by the rust is found near by the steel bar and converges the main crack. The distribution, composition and the developing of the rust at steel-concrete interface were studied by the microscopic methods of BSE, EDAX and Raman Microscope. By these methods, the rust distributes inhomogeneous and the ion element area was divided into zones evidently. From concrete to steel bar, the rust in turn is: initial rust zone, the mixture area of the rust and mortar about 20-50 μm in width and main containing the goethite (FeOOH); the secondary rust zone, high density area of ion element and about 10-20μm in width and being the mixture of hematite（Fe2O3）and magnetite(Fe3O4); the final rust zone, similar as the initial rust and being the mixture of hematite(Fe2O3)and goethite(FeOOH). According to the electrochemistry principle, the forming process of rust does not follow the iron oxidation procedure entirely but form middle production under the special situation of the condition interior concrete and the electric field outside. The extrusion action of concrete around steel bar makes the rust compressed and metamorphic and the secondary rust forming. So the process and the final state of this special corrosion production are the key parameters leading to the deterioration of concrete.